Thiabendazole gained commercial importance when Brown et al. (J. Am. Chem. Soc. 83, 1764 (1961)) reported that the compound prepared by reacting 4-thiazolecarboxamide with o-phenylenediamine using a polyphosphoric acid catalyst exhibited broad spectrum anthelmintic activity without adverse toxic effects. The reaction described by Brown et al. was conducted at a temperature of 250.degree. C. and resulted in a 64% yield of thiabendazole. Thiabendazole continues to be extensively used for the treatment and/or prevention of helminthiasis in livestock. Thiabendazole is, also, a systemic fungicide widely used for pre-and post-harvest spoilage control of raw agricultural commodities. Because of the commercial importance of thiabendazole, the chemical literature is replete with various other synthetic routes aimed at producing this pharmacologically and fungicidally active compound in high yield and high purity. Prior art processes for preparing thiabendazole suffer from inherent drawbacks and inconveniences, such as low yields, additional purification steps, long reaction times, environmentally undesirable organic solvents and high pressure reaction conditions. Hence, it would be a significant improvement in the art, if thiabendazole could be efficiently prepared under mild conditions in high yield and in the purest possible form for agricultural and pharmaceutical use. It would also be an advantage, if it could be prepared using a simple, one-step, environmentally friendly process which is easily adapted to commercial scale production.